Based on the data of automatic weather stations in Beijing region, complementary rain gauge stations of Beijing Municipal Commission of Planning and Natural Resources, dualpolarization radar and windprofiling radar as well as the GPS data and ERA5 reanalysis data, we analyze the spatiotemporal characteristics, and causes of this July 2023 extremely torrential rain in Beijing. The results indicate that the accumulated precipitation and intensity of this event were significant, with an average accumulated precipitation of 331 mm and maximum precipitation 1025 mm at a single station, both of which broke historical records. The maximum hourly rainfall of 126.6 mm·h-1 ranked the second in history. This rainfall process can be divided into five stages, of which stage Ⅱ and stage Ⅳ of the rainfall accounted for 37.1% and 39.7% of the total process, being the primary phases. During stage Ⅳ, the corresponding jet stream intensity got much stronger, and the characteristics of high temperature and high humidity were more obvious. Terrain had a significant impact on the increase in rainfall. The rainfall increased most rapidly in the region between 100-300 m above sea level， and the maximum value appeared near the areas approximately at the altitude of 400 m. The average accumulated rainfall and maximum hourly rainfall in mountainous areas were 2.1 (3.0) times and 2.0 (2.7) times that of the plains during stage Ⅱ (stage Ⅳ), respectively. Stage Ⅱ was primarily influenced by the direct terrain lifting of lowlevel jet, while stage Ⅳ was characterized by the combined effect of terrain lifting and blocking convergence. On the morning of the 31 (stage Ⅳ), the coupling of the boundary layer convergence at the terminus and the lowlevel divergence at its entrance led strong ascending motions, promoting the massive βMCS to develop into a linear shape accompanied by a mesoγ scale vortex. This βMCS propagated northward along the western mountainous regions, forming a short train effect and triggering the shorttime extremely severe rainfall over 100 mm·h-1 at eight stations.